In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in both the manufacturing processes and after the materials are in service in finished products. Specifically, non-destructive evaluation (NDE) methods must assess the structural integrity of composite materials. This assessment detects inclusions, delaminations and porosities. Conventional NDE methods are slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One test method uses ultrasound and lasers. An external ultrasound source generates ultrasonic surface displacements in a work piece, and laser light from a detection laser is directed at the work piece. Ultrasonic surface displacements scatter the detection laser beam, and collection optics collect the scattered laser energy. The collection optics are coupled to an interferometer or other device, and data about the structural integrity of the composite structure can be obtained.
Laser ultrasound has been shown to be very effective for the inspection of parts during the manufacturing process. However, the wavelength of the detection laser used for detection of ultrasonic displacements is very often around 1 μm (1.064 μm for Nd:YAG lasers). This wavelength is very harmful to the human eye and exposure causes severe retinal damage. Because of this, strict laser safety precautions must be observed when using these laser detection systems. These precautions include engineering controls (physical barriers, interlocks, sensors) administrative controls (warning signs, policies, procedures), and personal protective equipment (PPE)(goggles, helmets, clothing). Engineering controls are often employed within industrial environments. Generally these controls involve placing the inspection facility in a closed room within a plant. This solution confines laser ultrasound inspection to a few locations in a plant for manufacturing. For other applications, like in-service inspection, a closed-room solution might not be practical or possible.
Different wavelengths of light have different physiological effects and require different safety precautions. Replacing the existing detection laser source with a more eye-safe laser source may not be practical or cost-effective.
Therefore a need exists for an effective and economical means in which more relaxed laser safety constraints may be employed with these detection lasers.